Daylight modelling and measurement (T7)

Question 1

What human requirements should (day)lighting satisfy?

Answer 1

• (Day)lighting for performance/task
• (Day)lighting for appearance.
• (Day)lighting to ‘feel good’.
• (Day)lighting for information.
• Daylight to save energy on electric lighting?

Question 2

Illuminance

Answer 2

is a measure of the flux of light falling onto a surface (from all of the directions ‘above’ the surface). Units of illuminance are lux

Question 3

Luminance

Answer 3

is a measure of the intensity of light emitted from a surface per unit area in a given direction

Units of luminance are candela per square meter

Question 4

No sky line

Answer 4

Because the amount of sky visible at the workplane is a governing factor for general illumination, where obstructions are present it is common to estimate the no-sky line, i.e. that point on the workplane where the sky just ceases to be visible.

Question 5

British Standard BS8206

Answer 5

The average daylight factor is used as the measure of general illumination from skylight.
It is considered good practice to ensure that rooms in dwellings and in most other buildings have a predominantly daylit appearance.
In order to achieve this the average daylight factor should be at least 2%
If the average daylight factor in a space is at least 5% then electric lighting is not normally needed during the daytime, provided the uniformity is satisfactory.
If the average daylight factor in a space is between 2% and 5% supplementary electric lighting is usually required.

Question 6

You can convert from DF values
 to lux levels

Answer 6

• If you know two of the quantities from the equation, you can calculate the unknown, e.g. if the daylight factor at a point is 2.5% and the external horizontal illuminance is 8,000 lux, the internal illuminance at that point is calculated to be (8000 x 2.5) / 100 = 200 lux.
• However, the relation (i.e. equation) assumes standard overcast sky conditions.

Question 7

Properties of the daylight factor

Answer 7

• It was conceived as a means of rating a space independent of actually occurring daylight conditions.
• It makes no account of the sun.
• It is a relative quantity, i.e. a ratio/percentage of outside levels.
• Orientation alone has no effect on the daylight factor.
• Climate/location has no effect on the daylight factor.
• The average DF alone tells you nothing about the spatial distribution of the daylight factor.

Question 8

Average and Median Daylight Factor

Answer 8

• The median is the middle value after the data has been rank ordered from lowest to highest (or vice versa).
• The median DF informs on the spatial distribution of the DF - half of the area will be above this value, and half below.
• Consider the following example where the size/shape of the space and amount of glazing is fixed, but the distribution in glazing changes. The ADF calculated using the equation will be the same for both cases.

Question 9

better daylight in buildings and higher daylight factors

Answer 9

not equal to each other

Question 10

Education Funding Agency (UK)

Answer 10

In 2013 the Education Funding Agency made climate-based daylight modelling (CBDM) and the useful daylight illuminance (UDI) metric a mandatory requirement for the evaluation of designs submitted for the Priority Schools Building Programme (PSBP) ~£15 billion

Question 11

reactions to CBDM

Answer 11

• The average daylight factor is sufficient
• Climate-based metrics are unproven
• Daylight factors can be verified by measurement
• CBDM is too complex / difficult / unreliable
• Horizontal metrics and room appearance
• Too little daylight in the PSBP requirement

Question 12

Daylighting


Research | Enterprise | Impact

Answer 12

“Impact from research is defined as an effect on, change or benefit to the economy, society, culture, public policy or services, health, the environment or quality of life, beyond academia”
 REF2021